Conventionally, there has been an inkjet method of forming an image on a recording medium by attaching inks, which are recording materials (color materials), to the recording medium, as a recording method of recording, for example, a character and an image onto a recording medium such as a recording sheet or a film.
Widely used types of ink for inkjet recording apparatuses are a dye ink, which contains dye as a color material, and a pigment ink, which contains pigment as a color material. The pigment ink contains, for example, resin, water, and a color material, and has such a characteristic that solid contents thereof such as a color material and resin are easily deposited on the surface of a recording medium, compared to the dye ink. FIG. 1 schematically illustrates pigment color materials deposited on a recording medium. Further, forming an image on a recording material using the pigment ink leads to a phenomenon of coloring of specular reflection light which is light reflected by the formed image. More specifically, when an image formed by this kind of recording apparatus is placed under a light source such as a spotlight, although the spotlight emits achromatic light, this light turns into colored specular reflection light after being reflected on a recording medium. For example, in a color image, a region with a cyan ink laid on a large part of the region tends to look colored magenta, while a monochrome image tends to look colored yellow as a whole. Further, such coloring of specular reflection light tends to change in an iridescent manner according to a change in an ink amount (discharge amount) used in a predetermined area of an image. Occurrence of coloring of specular reflection light results in deterioration of the image quality due to a difference between the color of the specular reflection light and the color of the diffused light.
Now, a method of measuring coloring of specular reflection light (Japanese Patent Application Laid-Open No. 2006-1.77797) will be now briefly described with reference to FIG. 2. A measurement sample 201 is irradiated with light by a light source 202 from a predetermined angle, and the specular reflection light reflected by the measurement sample 201 is detected by a light receiver 203. The light receiver 203 detects tristimulus values XxYxZx in the International Commission on Illumination (CIE) XYZ color system. The degree how much the specular reflection light is colored is indicated as color saturation C*, which is expressed by a*b*h in the CIE L*a*b color system, based on a difference between the detected XxYxZx and tristimulus values XxYxZx of a sample that does not cause bronzing (for example, a black polished glass plate in which the wavelength dispersion of a reflective index is small). Less colored specular reflection light results in reduced C*, and C* becomes zero for a sample that does not cause coloring of specular reflection light (in other words, C* is positioned on the origin point on the a*b* plane).
Bronzing and thin-film interference are known as reasons that specular reflection light is colored as mentioned above.
Bronzing is a phenomenon that occurs due to the wavelength dependency of reflection on an interface of a formed image. It is known that each ink has a unique color to which the color of the ink is changed by a bronzing phenomenon. For example, specular reflection light is colored magenta in a region where an image is formed by a cyan ink. Japanese Patent Application Laid-Open No. 2008-236219 discusses that, when an image is formed on a recording medium using a plurality of recording materials, occurrence of bronzing is prevented by determining a recording order in which the color materials are laid, in such a manner that a recording material having smaller tristimulus values indicative of bronzing is overlaid on a recording material having larger tristimulus values.
However, according to the method discussed in Japanese Patent Application Laid-Open No. 2008-236219 it is impossible to completely overlay a color material on another color material in an image region using a color material having large tristimulus values indicative of bronzing more than a color material having small tristimulus values indicative of bronzing, and therefore, this method is less effective in such a case. Especially in a highly color-saturated image region, this ineffectiveness is more remarkable due to heavy use of a single recording material. In other words, this conventional method leaves much to be improved.
Another possible measure against coloring of specular reflection light is a method of using a clear ink, which is an ink containing no color material, as a recording material laid on the outermost surface of a recording medium, as illustrated in FIG. 1. A transparent clear ink has significantly small tristimulus values indicative of bronzing, and does not affect color development. Therefore, a clear ink can be used in any image region, and is expected to reduce coloring of specular reflection light more effectively.
However, this method results in a change in coloring of specular reflection light according to the discharge amount of a clear ink, since an optical path difference occurs in reflected light between an upper layer and a lower layer of a clear ink layer formed on a recording medium, and this optical path difference causes a thin-film interference.
This coloring of specular reflection light will be now described with reference to FIG. 3. FIG. 3 schematically illustrates the result of measuring the coloring of specular reflection light when a clear ink is laid on a solid surface formed by a cyan ink while changing the discharge amount of the clear ink with use of the method discussed in Japanese Patent Application Laid-Open No. 2006-177797, and then plotting the measured values on the a*b* plane. The numbers on the graph indicate the discharge amount of the clear ink. This graph shows that the coloring of specular reflection light reflected by the solid surface formed by the cyan ink is located in the magenta color phase, and the coloring is rotated in the clockwise direction on the a*b* plane according to an increase in the amount of the clear ink. In this way, this measurement has revealed that recording a clear ink on a color ink does not necessarily reduce coloring of specular reflection light, and coloring varies depending on the amount of a clear ink.
Further, coloring also varies depending on the type of a color ink laid under a clear ink. For example, coloring caused when a predetermined amount of a clear ink is overlaid on a solid surface formed by a cyan ink is different from coloring caused when the same amount of the clear ink is recorded on a solid surface formed by a magenta ink. In other words, just recording a predetermined amount of a clear ink on a color ink cannot completely reduce coloring in reflection on an interface between the color ink and the clear ink.